8, 01737 Tharandt, Germany Received 5 July 2004; accepted 18 May 2005 Abstract – We studied the influence of the timing of direct sunlight on the growth of Picea abies seedlings in a fie
Trang 1DOI: 10.1051/forest:2005058
Original article
Morning, noon, or afternoon: does timing of direct radiation influence
the growth of Picea abies seedlings in mountain forests?
Peter BRANGa*, Stefanie VON FELTENa,b, Sven WAGNERc
a WSL Swiss Federal Institute of Forest, Snow and Landscape Research, Zürcherstrasse 111, 8903 Birmensdorf, Switzerland
b Current address: Institute of Environmental Sciences, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland
c Institut für Waldbau und Forstschutz, Technische Universität Dresden, Pienner Str 8, 01737 Tharandt, Germany
(Received 5 July 2004; accepted 18 May 2005)
Abstract – We studied the influence of the timing of direct sunlight on the growth of Picea abies seedlings in a field experiment on a
north-facing slope in the subalpine zone of the Swiss Alps Vertical walls were established to expose three-year-old P abies seedlings to direct
sunlight at different times of day (morning, noon, afternoon), and to diffuse light only, during two growing seasons The light treatments were chosen in order to simulate microsites in forest gaps with different orientations While the light treatments resulted in different daily soil temperature curves, they affected neither average growing season soil temperature nor the frequency of soil temperatures above 10 °C, which are assumed to be particularly beneficial for seedling growth Final seedling biomass was unaffected by the timing of direct sunlight, but lower
for seedlings receiving diffuse light only These findings suggest that the orientation of slit-shaped gaps in subalpine forests to promote P abies
seedling growth is unimportant
gap orientation / seedling growth / time of day of direct radiation / Picea abies / soil temperature
Résumé – Le matin, à midi, ou l'après-midi : le moment du rayonnement direct influence-t-il la croissance des semis de Picea abies en forêt de montagne ? Nous avons étudié si le moment du rayonnement direct pendant la journée influençait la croissance des semis de Picea
abies Cette recherche a été réalisée dans un site expérimental en pente exposé au nord dans la zone subalpine des Alpes suisses Des parois
verticales ont été érigées afin d’exposer des semis de P abies de trois ans au rayonnement direct à différents moments de la journée (le matin,
à midi, l’après-midi), et au rayonnement diffus seulement, et ceci pendant deux périodes de végétation Les microstations formées par les traitements d’exposition à la lumière devaient ressembler à des microstations en ouvertures diversement orientées en forêt Bien que ces traitements aient modifié l’évolution de la température du sol au cours de la journée, ils n’ont influencé ni la température moyenne du sol pendant la période de végétation ni la fréquence des températures du sol supérieures à 10 °C, un niveau supposé être particulièrement bénéfique
à la croissance des semis Le rayonnement direct n’a pas modifié la biomasse finale des semis, mais la biomasse des semis qui ne recevaient que du rayonnement diffus était plus petite Il résulte de cette étude que l’orientation des ouvertures en fente dans des forêts subalpines n’est
pas susceptible d’influencer la croissance des semis de P abies.
orientation des trouées / croissance des semis / moment de la journée avec rayonnement direct / Picea abies / température du sol
1 INTRODUCTION
In the upper montane and subalpine zones of Switzerland,
Norway spruce (Picea abies (L.) Karst) is the most abundant
tree species The continuity of the frequently pure stands is of
great importance, as they often protect settlements and
infra-structure against snow avalanches and rockfall Since many of
these stands are currently 120 to 200 years old [13, 33], while
their lifespan is about 250–350 years [12], they need
regener-ation within the next 100 years Regenerregener-ation of these stands,
however, is delicate and may require several decades [8, 34]
To induce regeneration on steep-slope protection forests,
cutting slit-shaped gaps has often been recommended [10, 11,
20, 27], and has been increasingly practised since about 1990 [47], in particular on north-facing slopes The gaps allow direct sunlight to warm the rooting zone of the seedlings, which is important for root growth [9] and photosynthesis [17, 18] and thus for successful establishment These gaps should be ori-ented obliquely to the contour line of a slope to avoid avalanche formation, while providing for sufficient direct sunlight Man-agers can largely influence the time when direct light reaches the forest ground in the gaps by their size, shape and orientation [10] Therefore, if the timing of sun patches affects seedling growth, this should influence management decisions
P abies seedlings establishing in small gaps of subalpine
forests do not achieve root depths of approximately 5 cm before
* Corresponding author: brang@wsl.ch
Trang 2their 3rd or 4th year [9] Soil temperature within this layer limits
root growth [4, 16], which starts at a temperature of 8 and 12 °C
[42] and increases up to about 20 °C [41, 42] Site factors
affect-ing near-surface soil temperatures are air temperature, direct
sunlight, vegetation cover [3, 16, 28], snow cover [3], and soil
properties such as water content [38]
While direct sunlight (for instance, potential direct radiation
during 1–2 h per day in June) has been shown to be essential
for successful seedling growth in upper montane and subalpine
forests [9, 11, 20, 21, 27, 31], little is known about whether the
time of day of exposure to direct sunlight matters Bischoff [6]
presumed afternoon light to be particularly advantageous under
wet-cool conditions (north slopes, oceanic climate), and
morn-ing light under moderately cool and dry conditions (south
slopes, continental climate), as the latter dries out the soil to a
lesser extent In a field experiment with artificial gap
environ-ments, Wayne and Bazzaz [46] investigated the effect of
morn-ing (8:50–13:20) versus afternoon (11:40–16:50) sun patches
on birch seedling biomass, and found no significant effect of
the timing of light incidence However, this experiment was
conducted at about 350 m above sea level in the temperate zone
where temperature is not supposed to be a limiting factor for
the growth of many species, including P abies [34] In contrast,
afternoon light seemed more beneficial to P abies seedling
growth than morning and noon light on north and south slopes
in a sowing experiment in Swiss mountain forests [9]
Not only the amount of radiation, but also its quality, in
par-ticular the red:far red ratio, can affect seedling growth [5, 15,
26, 45] However, whereas shade intolerant tree species like
Pinus radiata show increased height growth under the low
red:far red ratios which can occur in closed stands, shade
tol-erant P abies saplings tend to form an umbrella-like,
flat-shaped crown [23, 36] Moreover, the red:far red ratio near the
ground seems to be unaffected by a canopy of mature P abies
[2] We did not, therefore, include the red:far red ratio in our
study
By means of a field experiment, we tested the hypothesis that
the time of day of exposure to direct sunlight affects upper soil
temperature and growth of P abies seedlings on steep north
slopes Under the moist conditions on such slopes, afternoon
direct light was hypothesized to be most beneficial because at
this time of day the soil has already been warmed up by higher
air temperatures [9] Thus, the addition of direct light in the
afternoon should induce a more prolonged shift of soil
temper-atures into a range favourable for root growth than does direct
light in the morning or at noon In addition, we investigated the
strength and time lag of the response of soil temperature to
inci-dent direct sunlight
2 MATERIALS AND METHODS
2.1 Experimental setup
The experimental site was situated on a north slope in the
Calfeisen-tal (Switzerland, 9° 18’ 30’’ / 46° 55’ 15’’), in the subalpine zone The
site is at 1725 m above sea level, and has a slope of 50% Scattered
P abies saplings from natural regeneration were present on the site,
but about 50 m higher up only isolated Pinus cembra trees remained.
This indicates timberline conditions for P abies, which we considered
desirable since potential effects of direct sunlight on seedling growth are probably more pronounced on such a cool site than at a warmer site at lower altitudes
Two-year-old seedlings of P abies from a high altitude provenance
(Rüeschegg (Gantrisch), Canton of Fribourg, Switzerland, 1620 m above sea level, northern aspect) were potted pair-wise in a standard nursery substrate with a light standard long-term fertilization (33 l Toresa, 8 l peat, 90 g Osmocote Plus, 90 g horn meal) in April 1999 The round plastic pots (20 cm high, 20 cm wide) were chosen so as
to be large in comparison to seedling size to reduce the risk of drought
We reduced the variation in initial height of the seedlings by discarding very small and very large seedlings Initial heights were 163.5 ± 3.2 mm
(mean ± standard error of the mean, n = 277) Before final transfer to
the field site, the seedlings were kept nearby on 1450 m above sea level for acclimation for one year At the start of the experiment in July 2000, the seedlings were three years old
On the experimental site, vertically placed wooden walls were attached to poles to provide spots with different light exposures (Fig 1) The walls were 2.5 m long and 1.3–2.3 m high They were erected in summer 2000 and temporarily lain flat during winters to pre-vent damage from moving snow The poles were left standing The
Figure 1 (A) Experimental setup of a single replicate a = treatment
‘afternoon’; c = treatment ‘control’; m = treatment ‘morning’; n = treatment ‘noon’ In the field, the pots were actually buried, and the
ground surface in the pots was at the same level as outside (B) View
of the experimental site early in the morning All seedlings, which are planted downslope of the wooden walls, are still in the shadow
Trang 3dates of erecting and taking down the walls were 6–7 July and 25 October
in 2000, 11 June and 11 October in 2001, and 30 May and 30 October
in 2002 As the upper crowns of the seedlings increasingly received
unplanned direct light, the experiment was stopped in autumn 2002
Bud break of trees at this altitude starts around mid June
The planting spots were selected north (downslope) of the walls
using a horizontoscope, an instrument which enables rough estimation
of potential duration (without clouding) and timing of direct sunshine
in different months [37, 40] The four light treatments were: direct
sun-light in the morning, at noon and in the afternoon, and only diffuse
radiation These will be referred to as ‘morning’, ‘noon’, ‘afternoon’
and ‘control’ The period of exposure to direct sunlight in the three
treatments ‘morning’, ‘noon’, ‘afternoon’ was between 1.5 and 3 h
daily and chosen so as to ensure roughly equal amounts of incoming
light energy To avoid heat congestion near the planting spots, a gap
of 5–10 cm was left between the walls and the ground The 4 treatments
described above were replicated 10 times (10 blocks) resulting in a
total of 40 planting spots
Six seedlings were assigned to each of the 40 planting spots, four
of them pair-wise in a pot, two planted nearby in the soil (Fig 1) The
resulting split-plot design has 40 main units (the planting spots) and
80 subunits One subunit contains either 4 seedlings grouped into two
pots of two seedlings, or two seedlings outside the pots Multiple
seed-lings were planted per subunit in order to avoid missing data as a result
of seedling mortality However, out of the total number of seedlings
of 6 × 4 × 10 = 240, only four seedlings had died from planting by the
end of the experiment, each in a different planting spot
Forty additional seedlings were placed adjacent to the field
exper-iment to provide a second control group receiving full light all day long
(further referred to as ‘full’ treatment) Like the other seedlings, these
seedlings were either potted (32 seedlings) or outside pots (8
seed-lings) However, we did not include these seedlings in the statistical
analysis since this treatment was not replicated and was thus outside
the experimental design On seedlings that had suffered from frost
damage, we clipped multiple stems in order to have trees with a single
terminal leader at the beginning of the experiment in summer 2000
This treatment forced seedlings to allocate their resources to only one
leading shoot In each of the four light treatments, 30 to 43% of the
seedlings remained unclipped, on 28 to 40% one stem was clipped,
and on 13 to 20% two stems were clipped All seedlings were protected
against herbivory with a mesh wire Herbs and grasses which occurred
on the site were left to grow around the seedlings Their cover averaged
20% in summer 2001, while their height was 10–50 cm
2.2 Measurements
Potential direct radiation (for days without any clouding) was
esti-mated using hemispherical photographs taken with a Nikon® 8 mm f/
2.8 lens [29] Photographs were analysed according to Wagner [43,
44] using a macro based on the Optimas® software (Optimas 6.5.172,
Media Cybernetics, Silver Spring, USA) Time resolution of
compu-tations for the sun path was 2 min Potential energy input by direct
sun-light (further referred to as direct energy) was calculated for cloudless
weather conditions, and the diffuse site factor [44] used as a measure
of diffuse radiation
During the experiment, precipitation, global radiation, air
temper-ature and air humidity were measured continuously using an automated
weather station with a data logger (CR10AX, Campbell Scientific,
Leicestershire, UK), which was located in the control treatment with
full access to direct radiation In addition, 32 sensors (M-CS505,
Campbell Scientific) were used to continuously measure soil
temper-ature at a depth of 4 cm, with a 10 min-interval during summer and a
2-h interval during winter Twenty-eight sensors were placed in 3
rep-lications of the seedling experiment, with 6 or 8 sensors attributed to
each treatment Eight sensors were placed outside the pots, but all
oth-ers were inside pots Four sensors were installed in the treatment ‘full’
At the end of September 2000 we measured initial height (to the nearest mm) and basal diameter (0.1 mm, average of two values meas-ured crosswise) of each seedling After two growing seasons (end of August 2002) these parameters were remeasured, and, in addition, the terminal shoot length in 2002 was recorded (mm) Moreover, we meas-ured the below- and aboveground dry weight of 30 seedlings (6 from the four experimental treatments and 6 from the treatment ‘full’) col-lected in 2002 in order to estimate the biomass of all experimental plants using the parameters measured
2.3 Data analysis
To test if the light treatments actually did differ as expected, we compared the average daily direct energy input and the diffuse site
fac-tors per plot (n = 40) among treatments using analysis of variance.
A posteriori multiple comparison tests were Bonferroni-corrected For each treatment, we also compared soil temperatures during the grow-ing season from June to September We compared temperature aver-ages, and in order to account for situations with satisfactory root growth conditions for the subalpine zone, we calculated the amount
of °Celsius × min when soil temperatures exceeded 10 °C Since some data are missing in 2001, we used only data from 2002 for soil tem-perature comparisons Four sensors had to be excluded from the anal-ysis of soil temperatures because they deviated largely from the others
in a laboratory test of sensor performance
Our target parameter for seedling performance was final seedling biomass (Bfinal, g), which we estimated from the measurements of the
excavated seedlings using the following linear regression (n = 30, R2=
0.9242, p < 0.0001):
Bfinal = –4.811 + 0.232 × (BDfinal)2 + 0.035 × Hfinal – 0.073 × TSLfinal, where BD is the basal diameter (mm), H the height (mm) and TSL the terminal shoot length in 2002 (mm) of the seedlings We did not use biomass gain as a target parameter since no data on the basal diameter
at the beginning of the experiment were available, and estimating ini-tial biomass with seedling height only and calculating biomass gain
as the difference between final und initial biomass resulted in negative values for biomass gain for several seedlings
Initial height and final biomass values of seedlings in pots on one side, and of those outside pots on the other side, were averaged per
subunit (n = 80) Final biomass was first square-root transformed and
then analysed using a linear mixed effects model Light treatment and pot were fixed factors, replicate was a random factor, and direct energy, the diffuse site factor and initial seedling height were used as covariates Two unpotted seedlings, accidentally from the same planting spot, were excluded after an outlier analysis, which reduced the final
sample size to n = 79 The treatment ‘full’, which was not part of the
experimental design, was excluded from the linear mixed effects model The time lag between changes in soil temperature and changes in air temperature was calculated in an explorative way by maximizing the fit of a linear regression between soil temperature and air temper-ature as a function of varying time lags on clear days The time lag between the exposure to direct sunlight and soil temperature response was determined by comparing graphically the slope changes in soil temperature curves as a function of direct sunlight Data were analysed using SAS® software (SAS Release 8.02, the SAS Institute Inc., Cary,
NC, USA)
3 RESULTS 3.1 Weather conditions
Monthly mean air temperatures of the nearest weather sta-tion of MeteoSwiss (the Swiss federal weather service) in Elm
(965 m above sea level) correlate strongly (R2 > 0.95) with our
Trang 4in situ measurements for the same period (Fig 2) Thus, the
per-manent records from Elm were used to compare temperature
and precipitation during the experiment (2001–2002) with
long-term means (1959–2002) If the general rise in
tempera-ture since the 1950’s is taken into account, the mean growing
season temperature (June to September) in 2001 was cooler
than the expected mean by –0.6 °C, and the temperatures in
2002 were warmer by 0.4 °C
3.2 Performance of sunlight exposure treatments
The analysis of the hemispherical photographs showed that
the artificially created sunrises and sunsets at each planting spot
clearly differed in the four treatments (Fig 3) However, some
unplanned ‘sunrises’ and ‘sunsets’ occurred (not shown in
Fig 1), especially in the treatment ‘noon’ and around the
sum-mer solstice, since the shading walls were not high enough to
completely obscure the sun path In the treatments ‘morning’
and ‘afternoon’ this unplanned exposure of the seedlings to direct sunlight was almost negligible, as unexpected sunflecks occurred mainly early in the morning or late in the evening In contrast, in the treatments ‘noon’ and ‘control’ the input of unplanned direct energy was substantial (Fig 4) In the treat-ment ‘noon’, this was often due to prolonged planned sunflecks (and not additional unplanned sunflecks) In the treatment
‘noon’, the planned direct energy alone would have been much less than that received in the treatments ‘morning’ and ‘after-noon’ The unplanned direct energy thus made the treatments with direct sunlight more similar with respect to the direct energy received
The potential energy input of direct sunlight varied accord-ing to the type of light treatment (Fig 4, Tab I) Inputs were significantly higher in the treatment ‘afternoon’ than in the treatments ‘morning’ and ‘noon’ Seedlings in the ‘control’ treatment with no direct planned sunlight obtained significantly less direct energy than those in the other treatments, but clearly more than expected
Figure 2 Monthly average soil and air temperatures measured at the field site in the Calfeisental and air temperatures from the weather station
in Elm Soil temperatures measured at a depth of 4 cm
Figure 3 Planned ‘sunrises’ and ‘sunsets’ in each of the planting spots in treatments with planned exposure to direct sunlight Spots in the
control treatment are excluded
Trang 5The diffuse site factor was 0.80–0.83 in the ‘full’ treatment
with no shading at all (based on two hemispherical
photo-graphs) In the four experimental treatments (n = 10 in each),
it was higher in the treatments ‘control’ (0.53 ± 0.01, mean ±
standard error of the mean) and ‘noon’ (0.53 ± 0.02) than in the
treatments ‘afternoon’ (0.45 ± 0.02) and ‘morning’ (0.46 ± 0.01)
This treatment effect was significant (p < 0.0001, ANOVA,
model not shown)
Differences between treatments in mean soil temperature at
a depth of 4 cm during the growing season 2002
(June-Septem-ber) were small and their ranking inconsistent between months
All treatments showed an average between 9.1 and 9.3 °C,
‘noon’ being the warmest treatment followed by ‘afternoon’
(–0.1 °C), ‘morning’ and ‘control’ In the second control
receiv-ing full light, soil temperatures averaged 10.5 ° C, and were thus
about 1.2 °C warmer than in the four other treatments Soil
tem-peratures exceeding 10 °C were most frequent and pronounced
in the ‘morning’ treatment (in 2002: 42% of the value of
276’359 °C min recorded in the “full” treatment), but ‘noon’
(39%) and ‘afternoon’ (38%) treatments were very close, and
exceedances were clearly higher than in the ‘control’ treatment
(34%) Daily temperature amplitudes on July 12, which was selected as an example of a sunny day, were also similar among
treatments and ranged between 3 and 4 °C (n = 3 to 6 sensors
per treatment) In the treatment with full light, daily amplitudes
of the same day were considerably larger (12 °C, n = 2)
Max-imum differences between sensors in the ‘full’ treatment and sensors with no direct light at a certain time were up to 10 °C
No differences in the average annual or daily soil tempera-tures were detected between the root zone inside and outside the pots In the pots, however, the daily amplitudes of soil tem-peratures were larger As a result, the soil temtem-peratures outside the pots exceeded 10 °C less often than those inside the pots (frequency of exceedances ~ 91% of those recorded in the pots)
3.3 Seedling growth
After three growing seasons, initial stem height was the most important determinant of seedling biomass (Tab II) Seedlings that were already tall when planted also had a large biomass at the end of the experiment This relationship was strongest in the
‘control’ and weakest in the ‘noon’ treatment (significant inter-action term: Initial height × Light treatment)
Seedling biomass was significantly affected by the direct sunlight treatments (Tab II) However, none of the treatments
‘morning’, ‘noon’ or ‘afternoon’ was significantly more beneficial for seedling growth than any other (Fig 5) The treatment effect was mainly due to the smaller final biomass of seedlings in the
‘control’ treatment, which was 9.5 ± 1.2 g (mean ± standard error) in comparison to 10.0–11.0 ± 0.9–1.3 g in the other treat-ments This effect was significant in the mixed effects model
(Tab II, p = 0.0322), although it is barely visible in Figure 5.
Furthermore, the seedlings in pots had significantly larger biomass (mean 10.9 g) than those planted directly in the soil (mean 8.7 g) There was a marginally significant tendency
Table I Analysis of variance of mean direct light energy received
per plot (n = 40, 10 in each treatment) and some associated a priori
(*) and a posteriori (**) contrasts, Bonferroni corrected with α = 0.05
Contrast ‘control’ vs other light
treatments
1 27 35.51 < 0.0001*
Contrast ‘afternoon’ vs ‘morning’ 1 27 0.0168**
Contrast ‘afternoon’ vs ‘noon’ 1 27 0.0936**
Figure 4 Planned and total (including unplanned) energy inputs
(daily averages) from direct sunlight for each treatment Error bars
represent standard errors of means
Table II Linear mixed effects model of seedling biomass by the end
of the experiment (n = 79) Effects marked with asterisks (*) are
ran-dom
*Replicate × light treatment
1.48 0.0689
Main plots (n = 40)
Mean daily radiant energy
Subplots (n = 80)
Initial height × light treatment
Contrast light treatment
‘control’ vs other treatments (a priori)
Trang 6(p = 0.0631) for the mean daily energy input from direct sunlight
to negatively affect seedling biomass (parameter estimate for
the influence of potential direct energy (kJ m–2) from May to
September on biomass: –0.00025 × direct energy ± 0.00013)
This is consistent with the small seedling biomass found in the
“full” treatment (8.5 ± 1.5 g in 2002), which is less than the
bio-mass reached in the four experimental treatments (Fig 5) The
seedlings in the ‘full’ treatment received a daily average of
9.000 kJ m–2 potential direct energy input and thus four to ten
times as much potential energy as the seedlings in the other four
treatments In contrast to the almost significant effect of direct
radiation on seedling biomass, the diffuse site factor was clearly not significant and was therefore eliminated from the linear mixed effects model
3.4 Light energy and soil temperature
Incident direct sunlight affected the soil temperature at a depth of 4 cm with a time lag of about 40 min, resulting in the different treatments having typical temperature curves The response of the soil temperature to changes in air temperature, which was derived from regressing soil temperature in the ‘con-trol’ treatment on air temperature using varying time lags for
two sensors on clear days, was more lagged, by 3–4 h (Fig 6,
maximum values of the two curves) The best regressions for
each of the two sensors, with a time lag of 3 and 4 h, had an r2
of 0.95
4 DISCUSSION
4.1 Performance of light treatments and effects on soil temperature
An experiment similar to ours, including the use of shading walls to investigate the impact of the timing of direct sunlight
on seedling growth, was used by Wayne and Bazzaz [46] in a temperate forest (Harvard forest, 350 m above sea level) To our knowledge, however, our study is the first of this kind to
be carried out within the subalpine forest zone, where site con-ditions and limitations are very different from those prevailing
in Harvard Forest
In our experiment, the amount of potential energy from direct sunlight reaching the soil surface was slightly overesti-mated for two reasons First, the fish-eye photos were not taken
at ground level, but about 12 cm above ground due to the size
Figure 5 Average final seedling biomass per treatment Data from
seedlings in pots and outside were pooled Error bars represent
stan-dard errors of means
Figure 6 Soil temperature curves on sunny days typical for each treatment, recorded on August 14 in 2001 This day was chosen (instead of
July 12) because unplanned direct energy input did not occur any more at this date, which is sufficiently distant from summer solstice
Trang 7of the fish-eye equipment, and second, there was some ground
vegetation This overestimation would have been greater during
the times of day when the solar angle was low, i.e in the ‘morning’
and ‘afternoon’ treatments Nevertheless, soil temperatures did
respond to the timing of direct light and thus differed between
treatments within the course of one day (Fig 6), consistent with
an earlier study [27] Moreover, some ground vegetation is
usu-ally present in real slit-shaped gaps In this respect, the
envi-ronment created in our experiment is close to real forest
habitats
The experimental treatments were successful in mimicking
the influence of direct radiation in forest gap environments at
different times of day However, this was less the case with
dif-fuse light Difdif-fuse site factors (0.45–0.55) exceeded values
found in real slit-shaped forest gaps by 50–100% On a steep
north-facing slope, Frehner [21] found diffuse site factors of
0.08–0.40 in small and some large gaps On sun-exposed spots
in small elongated openings, which our study tried to mimick,
values of 0.2–0.3 seem more realistic The additional diffuse
light may have weakened the effects of direct radiation on
seed-ling growth It is not clear to what extent the walls created a
similar microclimate to that in gaps with respect to air humidity,
air temperature and wind speed We assume that the differences
among treatments would have been more pronounced if treatments
had been applied more rigorously, i.e with no unplanned
sun-light at all, with more realistic (lower) levels of diffuse
radia-tion, and for a longer time span Despite these limitations, our
experiment clearly mimicked normal exposure to direct
radia-tion at different times of day
The time lag of 3–4 h between maximum air temperatures
and maximum soil temperatures at a depth of 4 cm in the control
treatment is in agreement with a soil temperature model built
by Hares and Novak [24, 25] They measured and simulated soil
temperatures on agricultural soils at a depth of 1 and 10 cm and
found that the temperature maximum at 10 cm depth was
approximately 4 h behind that at 1 cm depth If the reference
curve is measured in the free air as in our experiment and not
at 1 cm depth, the time lag at 10 cm depth would amount to a
time interval somewhat above 4 h
Neither the average growing season soil temperatures nor
exceedances of 10 °C varied much among our light treatments
The short energy pulses in the treatments with direct sunlight
did not result in higher mean soil temperatures during the whole
growing season than in the ‘control’ treatment This should be
seen in the context of an average clouding of about 60–70% on
the site in the years 2001 and 2002 during summer [14] The
direct energy input in the treatment ‘afternoon’, which we
hypothesized to exhibit the highest soil temperatures, was
higher than in the other treatments (Fig 4) This did not,
how-ever, result in higher average soil temperatures during the
growing season in this treatment, nor in more frequent and
pro-nounced exceedances of 10 °C So, not surprisingly, there was
also no significant increase in seedling growth
Even in the treatment ‘full’, the average seasonal soil
tem-peratures at a depth of 4 cm exceeded those in the control
(with-out planned direct sunlight) by only ab(with-out 1.5 °C At any time
of day, the difference between the ‘full’ and the other treatments
did not exceed about 10 °C (on July 12 in 2001, Fig 6) Sunlight
exposure exceeding four hours daily occurs only exceptionally
in small forest gaps on north slopes [9, 10] For this reason and given the small differences in average soil temperature at 4 cm depth between treatments (see above), such differences in real gaps are likely to be similar to or even smaller than those found
in our experimental treatments On clear days, Brang ([9], Fig 24) found, between 10:00 a.m and 6:00 p.m., maximum differences in average surface temperatures of 2 °C between microsites receiving different amounts of direct sunlight in
small gaps in subalpine P abies forests In contrast, direct
radi-ation can strongly influence temperatures at the soil surface in high-elevation environments [3], and instantaneous values at
1 cm depth [27] These results suggest that, given an interme-diate direct energy input, the time of day of exposure to direct
light is unlikely to influence the growth of P abies seedlings
by a soil temperature-root growth path
There are other factors that might have more effect on soil temperatures than the timing of direct sunlight In a field exper-iment in subalpine forests of south central British Columbia, Coates [16] found that, throughout the snow free period, soil temperatures at 10 cm depth were about 5 °C higher in soils where competing vegetation was removed than in soils beneath undisturbed vegetation Smaller, but still notable differences due to vegetation cover were found in an experiment in Quebec [28] Alexander [1] and Brang [9] both recorded higher soil sur-face temperatures on soils with a humus layer than on mineral soils In the latter case, in gaps on a subalpine north slope, tem-perature maxima differed by 6 °C
4.2 Seedling growth
Soil temperature has been shown to be a limiting factor for seedling growth in subalpine forests [3, 4, 16, 41], especially under moist conditions [9, 11, 27] The minor growth of seed-lings in the ‘control’ treatment than in the treatments receiving direct sunlight (Fig 5) is consistent with earlier studies sug-gesting 1–2 h of potential direct sunshine per day to be crucial for seedling growth in subalpine environments [9, 11, 20, 21,
27, 31] However, our starting hypothesis that exposure to direct light in the afternoon would be most beneficial for seed-ling growth, since this would result in the most prolonged shift
of soil temperature into a zone for satisfactory growth condi-tions (> 10 °C), must be rejected Neither soil temperatures nor seedling growth were higher in the treatment ‘afternoon’ than
in the other treatments with direct sunlight
Seedlings in the treatment ‘full’, where the soil temperatures were highest, showed the lowest biomass gains of all seedlings receiving planned direct sunlight Thus, seedling growth seems
to be influenced by both beneficial effects of direct light on soil temperature and negative effects Negative effects include potential drought and low temperature photoinhibition The latter
is caused by excess energy under low air temperatures Egerton
et al [19] have shown that Eucalyptus pauciflora trees benefit
from a reduction in irradiance (seedlings sheltered by vertical screens transmitting 50% incident sunlight) when growth is limited by low temperatures because of low temperature
pho-toinhibition Similarly, Germino et al [22] found Picea engel-mannii germinants survived better on the north than on the
south sides of tree islands This would be in line with the almost significant negative effect of direct energy that was detected in our experiment
Trang 8In this study, we exposed seedlings to experimental
treat-ments during two complete growing seasons (2001 and 2002)
The first application of the treatments in summer 2000, which
became effective at the beginning of July only, is unlikely to
have influenced seedling growth measured two years later to a
large extent The short two-year period of treatment exposure
may have contributed to the small effects found However,
given the small size of the effects after two years of treatment
exposure, large effects after longer exposure are unlikely
Initial height had a highly significant effect on final seedling
biomass This effect has repeatedly been demonstrated, leading
to many efforts to eliminate it in experiments [28, 30, 32, 35,
39] It is not surprising that initial height was most influential
for seedling biomass in the ‘control’ treatment (significant
interaction: Initial height × Light treatment), as these seedlings
had the poorest growth with the lowest gain in biomass
A fertilization effect may explain why seedlings in pots grew
better than those planted in the soil The standard fertilizer
added to the pots might well have been more nutrient rich than
the in situ subalpine soil Moreover, seedlings planted in the soil
may have suffered from transplanting shock [7] A soil
tem-perature effect (more frequent exceedances of 10 °C in pots) is
a less likely cause of this pattern
Delucia and Smith [18] found a significant correlation
between minimum night temperatures and reductions in
pho-tosynthesis in Picea engelmannii at high elevations in the
Med-icine Bow Mountains of Wyoming (USA) However, we can
assume that soil temperature minima at night did not vary
between the light treatments applied in our study since all
treat-ments were applied in the open
5 CONCLUSIONS
Our study suggests that, while a certain amount of direct
sun-light enhances the growth of P abies seedlings in subalpine
environments, the timing of exposure to direct sunlight is less
important The pathways of influence of direct sunlight need
further study We found more evidence for positive than for
negative effects of sun patches on seedling growth, but both
effects were present The results of our study do not support a
soil temperature-root growth-total biomass gain pathway,
which has been previously hypothesized [11, 27] Our results
suggest that such effects are absent since sun patches of a few
hours daily have only a small effect on the average temperature
and degree-minutes above 10 °C in the root zone of seedlings
Treatment differences found in an experimental setting such
as ours have to be very large to be relevant for management
since, in real forest habitats of P abies seedlings, sun patches
will be less clearly delineated than in experimental gaps, but
vary greatly in space and time While the edges of an opening
will create general daily and seasonal sun patch patterns, local
edge permeability due to the spatial position of nearby trees will
blur these patterns and lead to frequent changes between sun
patches and shade on a micro-site [10], regardless of edge
ori-entation In a management context, this means that the
orien-tation of small forest openings is of minor importance for
regeneration performance in subalpine P abies forests on
northern aspects, and clearly less important than the effects of
competing ground vegetation [9, 27, 28] and rainfall intercep-tion [9, 11] as long as the minimum light requirements of the seedlings are met When designing gap size, shape and
orien-tation in subalpine P abies forests to promote regeneration,
decisions should be made on the basis of criteria other than direct sunlight, including ease of timber harvesting, avoidance
of avalanche formation (which prohibits cutting gaps parallel
to the slope), the presence of micro-sites such as nurse logs which promote regeneration, and the presence of advance regeneration [27]
Acknowledgements: We thank Anton Burkart for providing the
seed-lings, Gustav Schneiter for advice and help in running a climate sta-tion, Hans-Rudolf Roth for his help with the statistical analysis and Pius Schmid and several field assistants for their efforts during field work
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